Abstract: At present, the preparation of highly homogeneous polyester fibers is still a great challenge. The stability of melt flow in melt spinning process and the degree of non-uniformity of radial velocity distribution before extrusion affect the quality of subsequent spinning. In this paper, the flow process of polyester melt in microporous flow channel was studied by numerical simulation. The melt flow characteristics under different microporous structure of spinneret and different inlet velocity were analyzed. The non-uniformity coefficient related to melt velocity was proposed, and the influence mechanism of microporous flow channel structure parameters on the stability and uniformity of melt flow was further clarified. The spinneret structure corresponding to the optimum non-uniformity coefficient of melt flow is pointed out.
With the increase of inlet velocity, the variation degree of melt average velocity before and after extrusion first decreases and then increases, and the flow is the most stable at 1 m/s. The time pressure drop is more stable when the inlet velocity ranges from 0.5 m/s to 1 m/s, and is more conducive to the internal stress of the primary fiber. The increase of flow velocity increases the average shear rate in the free section and strengthens the melt orientation. However, when the inlet flow velocity is 2 m/s and 3 m/s, the maximum shear rate in the flow passage reaches 105 orders of magnitude, and the possibility of melt fracture is higher, which is not conducive to the flow stability. The inlet velocity has little effect on the non-uniformity coefficient of melt flow when the spinneret geometry is unchanged. The study shows that the inlet velocity of 1m/s should be selected.
When the die length-diameter ratio is 2 and 3, the average velocity before and after extrusion changes less, and the flow is more stable. When the length-diameter ratio is 2 and 3, the pressure drop is more conducive to the internal stress of the primary fiber. When the aspect to diameter ratio is 2,3,4, the average shear rate in the free section is high, that is, the melt orientation degree is high. However, when the aspect to diameter ratio is 2 and 4, there is still a maximum shear rate in the free section, which is easy to lead to melt fracture and is not conducive to flow stability. At the same time, as the melt is always in the fully developed section when the length-diameter ratio is increased, the thicker the boundary layer is, the more uniform the flow will be, and the non-uniformity coefficient will decrease. The results show that 3 spinneret is the best.
When the convergence Angle is 54° and 74°, the average velocity before and after extrusion changes less, and the flow is more stable. When the convergence Angle is from 54° to 96°, the pressure drop changes stably with the maximum difference of 14.29%, which is within the reasonable range. The average melt shear rate in the free section decreases with the increase of convergence Angle, and the degree of melt orientation decreases. However, when the convergence Angle is 54° and 74°, the maximum melt shear rate in the center of the flow channel is small, and the melt is not easy to break, and the flow stability is better. At the same time, because the convergence Angle changes the radial velocity component at the entrance of the die, the influence on the flow uniformity is obvious. The results show that the spinneret with convergence Angle of 74° is the best.

Key words: spinneret structure, melt spinning, non-newtonian fluid, numerical simulation, rheology,  , non-uniform coefficient

摘要： 熔体在喷丝板微孔内流动时的稳定性和流场分布的均匀性是后续决定纤维成型质量的关键，这对熔体挤出前后的速度差、流道内的剪切速率分布和口模段内径向速度均匀性提出了要求。利用计算流体力学（Computational fluid dynamics，CFD）技术对涤纶工业熔融纺丝中的聚合物微孔挤出过程进行了数值模拟，得到了聚酯（Polyethylene terephthalate，PET）熔体在微孔内流动过程的速度、压力和剪切速率分布，讨论了熔体挤出前后的速度差和剪切速率分布对熔体流动稳定性的影响。提出了评价口模段内熔体径向速度分布的流动非均匀系数，指出不同长径比和收敛角分别通过改变流动充分发展段长度和口模段入口处径向速度分量来影响流动非均匀性。研究发现非均匀系数随长径比的增大而减小，随收敛角的增大先减后增。综合分析结果表明，长径比为3，收敛角为74°的喷丝板最佳。

关键词: 喷丝板结构, 熔融纺丝, 非牛顿流体, 数值模拟, 流变学, 非均匀系数